This invention relates generally to a mixing unit and more particularly to a mixing unit adapted to supply a mixture of gaseous fuel and combustion air to a premix burner of the type used in, for example, industrial heating systems. A mixing unit of this general type is adapted to receive a supply of gaseous fuel and a supply of combustion air by way of separate supply conduits. The fuel and the combustion air then mix together in the mixing unit whereupon the mixture is delivered to the premix burner by way of a transfer conduit.
Several mixing arrangements have been commonly used for mixing the fuel with the combustion air. For example, one prior mixing unit utilizes flowing combustion air to draw fuel into a relatively long mixing venturi whereupon the fuel and the combustion air mix together as they flow through the venturi. Another prior mixing unit causes the combustion air to swirl as it flows through a mixing tube and provides for radially outwardly directed jets of fuel to mix with the swirling combustion air. Generally, these and other prior mixing units tend to be relatively long in order to achieve a homogenous mixing of the fuel and the combustion air.
In addition, prior mixing units tend to cause a relatively large pressure drop in the combustion air as the combustion air flows through the mixing unit. A blower typically supplies the combustion air to the mixing unit and provides the air pressure which is necessary to move the combustion air through the heating system. The power which is required to operate the blower is related, in part, to the pressure loss in the combustion air as the combustion air flows from the blower to the burner. In prior mixing units such as the venturi-type mixing unit or the mixing unit which causes the combustion air to swirl in the mixing tube, the loss in air pressure due to the process of mixing the fuel and the combustion air can account for a substantial portion, if not the major portion, of the total pressure loss in the heating system. This total pressure loss can become substantial in industrial heating systems which require a relatively large volumetric flow rate of combustion air. In such heating systems, the additional capacity which is necessary to accommodate the pressure drop in the combustion air can result in the need for a larger blower. Moreover, the electric power associated with this pressure loss can amount to a substantial expense in the operation of the heating system.